1. Field of the Invention
This invention relates to the field of gas analysis and, more particularly, to the control of the admission of samples and the control of subatmospheric sample pressures in the use of a mass spectrometer.
2. Description of the Prior Art
A mass spectrometer is an arrangement for sorting streams of electrified particles (ions) in accordance with their different masses by means of electric or magnetic fields which include a mass filter. It consists of a chamber to which the particles are caused to pass while being subjected to the coercing field, radio frequency or other means for establishing the field, means for receiving and detecting the arrival of particles after they have traversed the field and, a sampling system for applying the particles of material to be studied. The chamber must be maintained in a vacuum high enough such that it will result in a mean free path to the particles which is comparable with the distance they must travel for effective interaction with the fields. The fields are such that when particles of several different mass numbers are supplied to the chamber, only those of a particular mass number determined by the deflecting field are passed to be detected. All others are, in effect, rejected by the mass filter and never detected. If the low pressure in the chamber is to be maintained, the rejected particle must be removed from the chamber as rapidly as the particles are admitted. The field may be varied, however, so that particles of a number of predetermined masses, if present in the sample, reach the detecting means sequentially in an order determined by the field variation. This enables the mass spectrometer to determine more than one component in a given gaseous sample.
Mass spectrometers have been operated in continuous communication with a volume of gas whose composition is to be studied. Normally, the gas to be studied is maintained at atmospheric pressure (hereinafter referred to as a "sample gas" or as a "atmospheric gas" or "atmospheric sample"). In order to maintain the required mass spectrometer high vacuum, the continuous admission of the sample must be accompanied by continuous pumping or removal of an amount equal to that admitted. To avoid unreasonable pumping requirements, it is customary to provide a limiting input device between the volume of sample gas to be studied and the spectrometer chamber. In the prior art such input devices have included certain pressure dropping arrangements such as capillary tubes, porous elements and exceeding minute apertures. These arrangements continuously permit sample gas to enter the chamber and determine the rate of gas entry and hence the required pumping capacity necessary to maintain the desired vacuum.
For practical pumping rates, the volume of a suitable capillary tube or porous element is significant as a limitation on the minimum sampling interval since the entire content of the tube must be taken into the chamber and evacuated before any change in the composition of the gas volume outside the chamber can be detected. This, of course, limits the response of the device to changes in the sample makeup. Moreover, the composition of the gas reaching the chamber may not be the same as that of the volume being investigated because of differential absorption or adsorption, condensation in or on the passage surfaces, or the release or entrainment of components previously so extracted. Minute apertures are difficult to produce with dimensional predictability and, even if of a sufficiently small size, to enable and assist operate with reasonable pumping capacity, they are extremely subject to stoppage by foreign particles in the sample gas. This is a very serious defect where combustion products or possible air pollutants are the subject of the investigation. Pumped manifolds, which are also used, share the above defect of capillary tubes and considerably increase both the complexity of the equipment and their required capacity.
One such prior art system utilizing the two-flow restrictions positioned in the flow stream upstream of the entrance to the vacuum chamber which uses no valving is found in a U.S. Pat. to Riggle et al. No. 2,714,164 issued July 26, 1955. That patent is an example of the sample manifold technique.
Other, earlier attempts at valving techniques for vacuum chambers are found in Hahn et al., U.S. Pat. No. 3,675,072, issued July 4, 1972, which uses a complicated electromagnetic fast closing valve system for emitting samples to a cyclotron, Sodal et al, U.S. Pat. No. 3,895,231, issued July 15, 1975, which utilizes a piezo electric crystal operated needle valve to control these sample gases into a vacuum chamber of a mass spectrometer and Asmus, et al., U.S. Pat. No. 3,483,373, issued Dec. 9, 1969, which utilizes an intermediate airlock chamber.
A single orifice pulse sample system for a mass spectrometer is disclosed in a patent to Bursack, a coinventor in the present application, U.S. Pat. No. 3,992,626, issued Nov. 16, 1976, and assigned to the same assignee as the present application. While by means of that invention, the amount of sample gas introduced into the mass spectrometer can be controlled so as not to exceed the capability of the ion-getter pump and thus the overall pressure within the chamber may be maintained, the sample is still introduced in a definite pulse which results in certain fluctuation in the desired steady state within the high vacuum chamber. This requires the mass spectrometer operation to be coordinated in time with the pulses and causes large pressure fluctuations.